A prior art optical transmitter used for the wavelength-division multiplexed optical communication system has a construction as shown in FIG. 14, comprising a plurality of semiconductor lasers A of different oscillation wavelengths, the optical outputs of which are independently modulated either directly or externally for wavelength-multiplexed optical transmission. Up to date, an optical transmitter using a Fabry-Perot resonator or like optical resonator also has been developed. The optical resonator in this optical transmitter converts light from a light source to multiple mode light, which has a regular wavelength spacing and is modulated independently by a plurality of external modulators for the wavelength-multiplexed optical transmission.
In the wavelength-division multiplexed optical communication system, a bandwidth limitation to a range centered on the wavelength of 1,550 nm is imposed on an optical amplifier, which relays optical signal. For mass communication by providing a large number of optical signals in the limited range, it is necessary to prevent isolation deterioration of adjacent optical signal transmitted from the optical transmitter in long use by accurately controlling the optical signal wavelengths. However, in order to preclude wavelength variations due to the wavelength isolation deterioration and ensure stable oscillation wavelength of the light source semiconductor laser such as to meet the above requirements, highly advanced techniques are required, giving rise to an optical transmitter price increase. In the optical transmitter using the optical resonator, it is possible to use a semiconductor laser, which is subject to wavelength variations, as the light source without giving rise to the problem of the wavelength isolation deterioration. This is so because multiple mode light, which is free from wavelength variations and is modulated, can be obtained from the laser beam by a resonating action of an optical resonator. However, the multiple mode light generated by the optical resonator is peculiar thereto. This is undesired form that the standpoint of the free wavelength setting. For instance, it is difficult to set non-uniformly spaced-apart channels.